1. Field of the Invention
The present invention is directed to a device for optimizing the speed control of a subfractional horsepower motor such as those typically used in dental instruments.
2. Description of the Prior Art
Two conventional control devices of this type are known in the art and disclosed in DE 32 21 146. In one of these designs, the regulation of the speed is accomplished through compensation of the internal voltage drop of the motor. The motor current is measured dependent on the motor load. Given a reduction in speed as a result of a higher load, a higher motor current is measured. Conversely, the speed increases with a reduction in load, and consequently a lower motor current is measured. The measured difference in motor current is fed to a controller that adjusts the speed to the desired value. An advantage of this type of motor regulation is extremely quiet operation of the motor even into its upper speed range. One disadvantage however, is that the motor provides relatively low torque, especially in its lowest speed range. The increases in current are too slight to compensate by measurement; in contrast thereto, the collector losses are felt to a relatively large degree.
In the second conventional control device, the speed is calculated by measuring the voltage with the motor switched off. One advantage of this design is that it provides extremely good torque for the motor, even in its lowest speed range. The regulation can compensate the collector losses well, and as a result speeds having high torque can be regulated down to zero. One disadvantage of this control method is that rough running that can lead to vibration may occur under certain circumstances in the upper speed range. The reason for this is that system-deactivation of the motor is necessary for the voltage measurement.
One object of the present invention is thus to provide a device that makes it possible to operate a subfractional horsepower motor of the type primarily used for dental equipment in such a way that it can be optimally controlled in every desired speed range. The device will provide optimum speed control even with different load values in both the lower as well as the upper speed range.